Photoresists containing water soluble sugar crosslinking agents

ABSTRACT

A photoresist is provided. The photoresist comprises a polymer, a photoactive agent, and a crosslinking agent. The crosslinking agent comprises a water soluble sugar. The present invention also provides a method of making microelectronic structures.

FIELD OF THE INVENTION

The present invention relates to the lithography field, and moreparticularly to photoresists including crosslinking agents, and tocrosslinking agents for such formulations.

BACKGROUND OF THE INVENTION

Novolak/diazonaphthoquinone (DNQ) resists have been the most widely usedphotoresists in the manufacture of microelectronic devices such assemiconductors since their introduction in the 1960's. However, the everincreasing demand for high resolution imaging has driven themicrolithography industry towards the use of short wavelength radiation,which is not suitable for DNQ resists. In general, these radiationsources have much lower output power than the conventional 360 nm or 435nm mercury emission lines. The resist materials that have been designedfor use with conventional sources are noncatalytic in nature and theiruse in combination with the lower power radiation sources leads to adrop in throughput as the time required for exposure of each waferincreases. In order to alleviate this problem, resists with much highersensitivities have been developed. One approach to increasing thesensitivity is the use of a chemically amplified system, such as theacid-catalyzed thermolysis of t-butyloxycarbonyloxy (tBOC) protectedpoly(4-hydroxystyrene). See, J. M. J. Frechet, et al., Polymer 24:995(1983). The increased sensitivities of these catalytic imaging materialsrelative to conventional DNQ resists has enabled the implementation ofdeep-UV lithography and also contributed to advances in electron beamlithography.

In the last decade, a large number of new imaging materials based onchemical amplification have been designed. Most of the modificationscontaining t-BOC or t-butyl ester active groups afford either positiveor negative-tone images as a function of the choice of developer.Alternatively, crosslinking through electrophilic aromatic substitutionfollowed by aqueous base development has been used to createnegative-tone resists that afford non-swollen images.

There remains a need in the art for photoresists having both highresolution and high sensitivity. There also remains a need in the artfor photoresists which are useful in deep-UV image resolutiontechniques. Moreover, there remains a need in the art for photoresistcrosslinking agents capable of providing high resolution and sensitivityphotoresists.

SUMMARY OF THE INVENTION

The photoresist of the present invention comprises a polymer, aphotoactive agent, and a crosslinking agent. The crosslinking agentcomprises a water soluble sugar. Typically, the crosslinking agent is aC₃, C₄, C₅, C₆, up to a C₁₂ water soluble sugar. Preferably, thecrosslinking agent is a C₅, C₆, or C₁₂ water soluble sugar.

The present invention also provides methods of making microelectronicstructures. The inventive methods include the steps of: (a) forming aphotoresist on a microelectronic substrate, (b) irradiatingpredetermined sites of the photoresist to activate a photoactive agentat the predetermined sites, and to create exposed and unexposed sites ofthe photoresist, (c) heating the photoresist, and (d) removing matterfrom the unexposed sites. The method of the present invention alsocontemplates additional, finishing steps such as etching or selectivedeposition.

The foregoing and other aspects of the present invention are explainedin detail in the specification set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical illustration of the sensitivity cures fornegative-tone photoresists including water soluble sugar crosslinkingagents.

FIG. 2 is a scanning electron micrograph of 1 μm features of anegative-tone photoresist prepared according to the method of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method of making microelectronicstructures comprising the steps of (a) forming a photoresist asdescribed below, on a microelectronic substrate; (b) irradiatingpredetermined sites of the photoresist to create exposed and unexposedsites; (c) heating the photoresist; and (d) removing matter from theunexposed sites. As used herein, the phrase "exposed sites" refersgenerally to the predetermined sites of the photoresist which areirradiated according to the method of the present invention. As usedherein, the phrase "unexposed sites" refers generally to the sites ofthe photoresist which are not irradiated during the step of irradiatingpredetermined sites of the photoresist.

The photoresist includes a polymer, a photoactive agent, and acrosslinking agent. Suitable polymers for use in the present inventionwill be known to those skilled in the art. Preferably, the polymers willbe transparent in at least a portion of the ultraviolet region of theelectromagnetic spectrum. As used herein, the term "transparent" refersto a 0.001 mm thickness of polymer which essentially has a percenttransparency of not less than 50% in the wavelengths between about 245nm and about 440 nm. Preferably, a 0.001 mm sample of the polymer has apercent transparency of not less than 50% at one or more of thefollowing wavelengths: 248 nm, 254 nm, 313 nm, 365 nm, and 435 nm.Generally, polymers which are useful in the present invention aresoluble in aqueous base solution. Typically, suitable polymers includepolymers containing ethylenic unsaturation or aromatic rings. Examplesof useful polymers include, but are not limited to,poly(hydroxystyrene), novolak, poly(p-tert-butoxycarbonyloxy-styrene),polyvinylbenzoic acid, poly (2-hydroxyhexafluoropropyl styrene),copolymers of styrene and hydroxystyrene, copolymers of styrene andmaleimide as described in S. R. Turner et al., Polym. Sci & Eng.26(16):1096 (1986), the disclosure of which is incorporated herein byreference in its entirety, and mixtures or copolymers of any two or moreof the foregoing polymers. Preferably, the polymer ispoly(hydroxystyrene) or novolak.

Any suitable photoactive agent known to those skilled in the art may beemployed in the practice of the present invention. As used herein, theterm "photoactive agent" refers to a compound whose chemical compositionis altered upon exposure to radiation. Preferred photoactive agentsinclude photoacid generators. Photoacid generators produce acid uponexposure to radiation. Photoacid generators which are suitable for thepresent invention typically produce strong acids upon exposure toradiation. J. M. J. Frechet, Pure and Applied Chemistry 63(9):1239(1992), the disclosure of which is incorporated herein by reference inits entirety, describes various photoacid generators which are useful inthe present invention. Examples of suitable photoacid generators includebut are not limited to a wide variety of sulfonium and iodonium salts,such as triphenylsulfonium hexafluoroantimonate, triphenylsulfoniumtriflate, triphenylsulfonium hexafluorophosphate, triphenylsulfoniumhexafluoroarsenate, diphenyliodonium hexafluoroantimonate,diphenyliodonium triflate, diphenyliodonium hexafluorophosphate,diphenyliodonium hexafluoroarsenate and the like, halogenated aromaticcompounds such as trichloromethyl triazine, o-nitrobenzyl sulfonates,tris(methanesulfonyl-oxy)benzene, and arylnaphthoquinone-diazide-4-sulfonates. The photoactive agent may bepresent in an amount of between about 0.1 percent to about 10 percent orabout 10 percent to about 20 percent based on the weight of thephotoresist composition. Preferably, the photoactive agent is present inan amount of between about 1 percent to about 10 percent by weight ofthe photoresist.

The photoresist further includes a crosslinking agent. Suitablecrosslinking agents include water soluble sugars. Typically, the watersoluble sugars which are suitable in the method of the present inventioninclude C₃, C₄, C₅, C₆, up to C₁₂ water soluble sugars. All that isrequired is that the sugar be effective to crosslink the polymer.Suitable sugars include monosaccharides, disaccharides,oligosaccharides, and the like. Preferably, the water soluble sugarsuseful in the method of the present invention include C₅, C₆, and C₁₂water soluble sugars.

Suitable water soluble sugar crosslinking agents include but are notlimited to xylose, ribose, glucose, fructose, galactose, arabinose,mannose, sucrose, maltose, lactose and cellobiose. Preferably, thecrosslinking agent is xylose, glucose, or sucrose.

The crosslinking agent is typically present in the amount of betweenabout 1 percent to about 25 percent, based on the weight of thephotoresist. Preferably, the crosslinking agent is present in an amountof between about 3 percent to about 8 percent by weight of thephotoresist.

The negative-tone photoresist may be formed according to methods knownto those skilled in the art. Namely, the photoresist may be providedalone, as a sheet or film. More preferably, the photoresist is formed ona microelectronic substrate. Suitable substrates are known to thoseskilled in the art. For example, the photoresist may be formed as acoating on a monocrystalline silicon or other semiconductor substrate.Other suitable substrates include printed circuit boards or other secondlevel packaging substrates.

Suitable methods of coating the photoresist on the substrate will beknown to those skilled in the art. For example, the photoresist may becoated on a substrate using spin-coating techniques which are well knownin the art. Spin-coating is the preferred method of coating thephotoresist on a substrate, although alternative methods known to thoseskilled in the art are contemplated by the present invention as well.

The formed photoresist is then irradiated at predetermined sites using asuitable mask, to activate the photoactive agent at the predeterminedsites. Irradiation at predetermined sites creates exposed and unexposedsites on the photoresist.

Typical structures formed by this process, depending on thepredetermined sites irradiated, include various microelectronicstructures such as transistors or integrated circuit chips, boards, andthe like. Any suitable form of radiation known to those skilled in theart may be employed in the method of the present invention. Preferably,the form of radiation employed will be dependent upon the transmissionproperties of the polymer selected. That is, the polymer selected shouldbe essentially transparent to the wavelength of radiation which is to beemployed. Suitable forms of radiation include ultraviolet, infrared,deep-ultraviolet, electron beam, X-ray, and ion beam. Preferably, thephotoresist is irradiated with ultraviolet, deep-ultraviolet,electron-beam, X-ray, or ion beam radiation.

After radiation exposure, the photoresist is subjected to post-exposurebaking. Post-exposure baking involves heating the photoresist to asufficient temperature to provide the activation energy necessary toinitiate crosslinking via electrophilic aromatic substitution. Withoutwishing to be bound by any particular theory regarding the mechanismwhereby electrophilic aromatic substitution takes place, Applicants'current belief is that this design relies on the acid-catalyzedformation of carbenium moieties from latent electrophilic groups. Thecarbenium moieties are believed to be generated by the acid-catalyzeddehydration of the sugar crosslinking agent. Alkylation of the aromaticrings of the matrix polymer by the carbenium moieties is believed tolead to crosslinking. Chemical amplification arises as thephotogenerated proton that is consumed in the initial formation of thecarbenium species is regenerated in the subsequent aromatic substitutionprocess. For example, in the embodiment wherein the photoactive agent isa photoacid generator, the crosslinking is initiated by the acidcatalyzed dehydration of the sugar followed by crosslinking of theresulting unsaturated compound under the acidic conditions of theexposed sites.

The appropriate heating conditions for post-exposure baking arenecessary for the optimal performance of the resist. Improper heatingconditions may have a negative impact on the sensitivity of the resist.Typically, the photoresist is heated at between about 90° C. to about150° C. for between about 15 seconds to about 15 minutes. Preferably,the photoresist is heated at about 130° C. to about 140° C. for betweenabout 30 seconds to about 5 minutes, more preferably for about 1 minuteto about 3 minutes.

After heating the photoresist, matter is removed from the unexposedsites. The removal of matter from the unexposed sites may beaccomplished by any means known to those skilled in the art. Typically,matter is removed from the unexposed sites by subjecting the resist to adeveloping medium, in which matter from the unexposed sites issolubilized. Preferably, inasmuch as the polymers useful in the presentinvention are soluble in aqueous base, the photoresist is developed inaqueous base developing solution.

Several suitable developing techniques will be known to those skilled inthe art. According to one developing technique, the photoresist isimmersed and agitated in a bath of developing solution maintained at apredetermined temperature for a predetermined period of time. Accordingto a second developing technique, the developing solution is sprayedacross the surface of the photoresist. Yet another suitable developingtechnique is the puddle technique, whereby a fixed amount of developeris dispensed on the photoresist and after a period of time, thedeveloping solution is removed by directing a stream of deionized wateronto the developed photoresist. Other suitable techniques will bereadily determined by the skilled artisan.

The aqueous base developing solution may comprise any suitable basicsolution known to those skilled in the art. Examples of suitable aqueousbase solutions include but are not limited to solutions which comprisesodium hydroxide, potassium hydroxide, sodium carbonate, and varioustetraalkylammonium hydroxides such as, for example, tetramethylammoniumhydroxide, and tetrabutylammonium hydroxide. The aqueous base solutionsmay also include mixtures of any two or more of the foregoing basesolutions. Preferably, the aqueous base solution comprises atetraalkylammonium hydroxide, more preferably tetramethylammoniumhydroxide.

The method of the present invention is typically used to prepare anegative-tone photoresist, inasmuch as matter is removed from theunexposed sites during the development stage. One skilled in the artwill, however, appreciate that the method of the present invention isnot limited to negative-tone photoresists. For example, U.S. Pat. No.4,657,845 to Frechet et al., the disclosure of which is incorporatedherein by reference in its entirety, describes an image reversaltechnique which may be applied to the photoresists of the presentinvention to provide positive-tone photoresists. Other suitabletechniques of effecting image reversal, or otherwise providingnon-negative tone photoresists will be known to those skilled in theart.

The method of the present invention may also include additional steps,known to those skilled in the art. For example, the method of thepresent invention may also include etching the substrate to transfer thephotoresist image into the substrate. According to one preferredembodiment, etching is accomplished by plasma etching. One skilled inthe art will appreciate that other finishing techniques, such asselective deposition and others, are contemplated by the instantinvention.

The following examples are provided to illustrate the present invention,and should not be construed as limiting thereof. In these examples, mmeans meters, nm means nanometers, μm means micrometers, min. meansminutes, ° C. means degrees Centigrade, wt. % means weight percent, andMj/cm² means megajoules per centimeter squared.

Xylose and glucose were obtained from Aldrich Chemical Company, Inc.,and may be used without further purification. Infrared spectra wereobtained on a Nicolet FTIR/44 spectrometer. Resist film thickness wasmeasured on a Tencor Alpha-Step 200 surface profiler. Deep-UV exposureswere performed by contact printing either using a Canon HTG Systems IIIContact Aligner or an Optical Associates Inc. exposure system comprisinga low pressure mercury lamp with a shutter system, an intensitycontroller, and an exposure timer. Photon flux was measured using anOptical Associates Inc. 354 exposure monitor. The output of the mercurylamp was filtered through a 254 nm narrow bandwidth interference filterfrom Oriel Corporation. Varying dosages of light for deep-UV sensitivitymeasurement were obtained with a Series 1 Multidensity resolutiontarget, Ditric Optics Inc. Sensitivities reported are accurate to ±0.05mJ/cm². E-Beam exposures of the resist films were conducted with aCambridge Instruments Electron Beam Microfabricator 10.5/CS Scanningelectron microscope.

EXAMPLE 1 Resist Formulation and Processing

Resist formulations are listed in Table 1 below. Methyl cellosolve isused as the casting solvent for spin coating onto silicon wafers.Pre-exposure bake is done at 110° C. for 2 min. to give 1.0±0.05 μmfilms. All samples are post-exposure baked for 3 min. Solventdevelopment of the irradiated and postbaked resists is done by dippingthe wafer into a beaker of rapidly stirred 30-40% aqueous AZ312MIF™solution for 25-35 seconds. Unless otherwise stated, sensitivity data isobtained with 254 nm radiation.

                  TABLE 1                                                         ______________________________________                                        Sensitivity Testing of Resists                                                Formulated from Xylose and Glucose                                                       wt. % of                                                                      triphenyl- Post-exposure                                                      sulfonium  bake                                                               hexafluoro-                                                                              temperature,                                                                             Sensitivity,                                 Sugar      antimonate °C. mJ/cm.sup.2                                  ______________________________________                                        xylose (5 wt. %)                                                                          5         120        19.0                                         xylose (5 wt. %)                                                                          5         130        12.0                                         xylose (5 wt. %)                                                                          5         140        7.5                                          xylose (15 wt. %)                                                                         5         120        13.0                                         xylose (15 wt. %)                                                                         5         130        10.0                                         xylose (15 wt. %)                                                                         5         140        9.0                                          xylose (5 wt. %)                                                                         10         120        5.5                                          xylose (5 wt. %)                                                                         10         130        5.5                                          xylose (5 wt. %)                                                                         10         140        4.3                                          xylose (10 wt. %)                                                                        10         120        8.0                                          xylose (10 wt. %)                                                                        10         130        8.0                                          xylose (10 wt. %)                                                                        10         140        4.7                                          glucose (5 wt. %)                                                                        10         120        7.1                                          glucose (5 wt. %)                                                                        10         130        5.1                                          glucose (5 wt. %)                                                                        10         140        4.5                                          xylose (5 wt. %)                                                                         10         140        10.0 μC/cm.sup.2 *                        glucose (5 wt. %)                                                                        10         140        10.0 μC/cm.sup.2 *                        ______________________________________                                         .sup.2 *Sample exposed with electron beam radiation.                     

EXAMPLE 2 Sensitivity

Several different combinations of photoacid generator,poly(4-hydroxystyrene) and sugars are evaluated in 3-component resistsystems. The compositions and resist sensitivities with deep-UV andelectron beam radiation are summarized in Table 1 above.

While normal sigmoid shaped sensitivity curves (see FIG. 1, curve a) areobserved for resists containing 5 wt. % of sugar, higher loading ofsugar, results in "2-step" sensitivity curves (see FIG. 1, curve b). Theexact cause is not clear. IR spectra show a significant decrease in thexylose and glucose C-O bond stretching at 1050 cm⁻¹, suggesting theformation of the corresponding for furaldehyde.

EXAMPLE 3 Crosslinking

To confirm that the resist functions through crosslinking of the polymermatrix, the temperature at which the resist begins to flow is observedby optical microscopy. A resist including 5 wt. % xylose and 10 wt. %triphenylsulfonium hexafluoroantimonate is spin-coated onto siliconwafers to form 1 μm films. The radiation-exposed and postbaked samplesare heated on a hot stage and observed under an optical microscope todetermine the temperature at which the polymer matrix starts to flow.The temperatures recorded reveal the trend for the change that resultsfrom the irradiation and subsequent post-exposure bake steps. Theresults obtained are as follows: 160° C. for a radiation-unexposedsample; 190° C. for a sample exposed to 5 Mj/cm². Because crosslinkingof a polymer is know to increase its glass transition temperature, theseresults are in agreement with the mechanism of operation of this resist.

EXAMPLE 4 Imaging

In order to demonstrate imaging, a preliminary experiment using 248 nmprojection printing is performed with a resist including 10 wt. % xyloseand 10 wt. % triphenylsulfonium hexafluoroantimonate. FIG. 2 shows thescanning electron micrograph of 1 μm features obtained after aqueousbase development. These results demonstrate the versatility of theconcept of radiation-induced crosslinking via electrophilic aromaticsubstitution.

The foregoing is illustrative of the present invention and is not to beconstrued as limiting thereof. The invention is defined by the followingclaims, with equivalents of the claims to be included therein.

That which is claimed is:
 1. A negative-tone photoresist comprising, inadmixture, a polymer, a photoactive agent, and a crosslinking agentcapable of crosslinking said polymer, wherein said crosslinking agent isa C₁ -C₁₂ water soluble sugar.
 2. The photoresist according to claim 1,wherein said crosslinking agent is selected from the group consisting ofC₅, C₆, and C₁₂ water soluble sugars.
 3. The photoresist according toclaim 1, wherein said polymer is essentially transparent in at least aportion of the ultraviolet region of the electromagnetic spectrum. 4.The photoresist according to claim 1, wherein said polymer is soluble inaqueous base.
 5. The photoresist according to claim 1, said polymercontaining ethylenic unsaturation or aromatic rings.
 6. The photoresistaccording to claim 1, said polymer containing aromatic rings.
 7. Thephotoresist according to claim 1, wherein said polymer is selected fromthe group consisting of poly(hydroxystyrene), novolak, polyvinylbenzoicacid, poly(2-hydroxyhexafluoropropyl styrene), copolymers of styrene andhydroxystyrene, copolymers of styrene and maleimide, and copolymers ormixtures thereof.
 8. The photoresist according to claim 1, wherein saidpolymer is poly(hydroxystyrene).
 9. The photoresist according to claim1, wherein said polymer is novolak.
 10. The photoresist according toclaim 1, wherein said photoactive agent is a photoacid generator. 11.The photoresist according to claim 1, wherein said photoactive agent isselected from the group consisting of sulfonium or iodonium salts,halogenated aromatic compounds, o-nitrobenzyl sulfonates,tris(methanesulfonyloxy)benzene, and arylnaphthoquinone-diazide-4-sulfonates.
 12. The photoresist according toclaim 1, wherein said photoactive agent is selected from the groupconsisting of triphenylsulfonium hexafluoroantimonate,triphenylsulfonium triflate, triphenylsulfonium hexafluorophosphate,triphenylsulfonium hexafluoroarsenate, diphenyliodoniumhexafluoroantimonate, diphenyliodonium triflate, diphenyliodoniumhexafluorophosphate, and diphenyl iodonium hexafluoroarsenate.
 13. Thephotoresist according to claim 1, wherein said crosslinking agent isselected from the group consisting of xylose, ribose, glucose, fructose,galactose, arabinose, mannose, sucrose, maltose, lactose, andcellobiose.
 14. The photoresist according to claim 1, wherein saidcrosslinking agent is xylose.
 15. The photoresist according to claim 1,wherein said crosslinking agent is glucose.
 16. The photoresistaccording to claim 1, wherein said crosslinking agent is sucrose. 17.The photoresist according to claim 1 coated on a substrate.